1. Field of the Invention
The present invention relates to an automatic transmission which changes rotations of an input shaft into multiple gear speeds transmitted to an output shaft via a plurality of planetary gear mechanisms disposed in a transmission case.
2. Description of the Related Art
Conventionally, there has been known an automatic transmission (for example, refer to Japanese Patent Laid-Open No. 2005-273768) which performs 8 forward gear speed changes via a first planetary gear for inputting, two planetary gears for gear change and six engagement mechanisms.
The first planetary gear in the automatic transmission disclosed in Japanese Patent Laid-Open No. 2005-273768 is of a double-pinion planetary gear mechanism comprised of a first sun gear, a first ring gear, and a first carrier pivotally supporting a pair of first pinions, which are intermeshed with each other with either one intermeshed with the first sun gear and the other one intermeshed with the first ring gear, in such a way that the pair of first pinions can rotate and revolve freely.
In the first planetary gear, the first sun gear is equivalent to a fixing element fixed to a transmission case, the first carrier is equivalent to an input element coupled with an input shaft, and the first ring gear is equivalent to an output element configured to decelerate a rotation speed of the first carrier equivalent to the input element and output the decelerated rotation speed.
The two planetary gears for gear change are of a Ravigneaux planetary gear mechanism comprised of a second sun gear, a third sun gear, a second ring gear integral with a third ring gear, and a second carrier pivotally supporting a pair of second pinions, which are intermeshed with each other with either one intermeshed with the second sun gear and the second ring gear and the other one intermeshed with the third sun gear, in such a way that the pair of second pinions can rotate and revolve freely.
In the Ravigneaux planetary gear mechanism, the second sun gear is equivalent to a first rotation element, the second carrier integral with the third carrier is equivalent to a second rotation element, the second ring gear integral with the third ring gear is equivalent to a third rotation element, and the third sun gear is equivalent to a fourth rotation element in the order of distances relative to gear ratios in a velocity diagram.
The engagement mechanisms are comprised of a first engagement mechanism releasably coupling the first ring gear equivalent to the output element of the first planetary gear with the fourth rotation element composed of the third sun gear, a second engagement mechanism releasably coupling the input shaft with the second rotation element composed of the second carrier, a third engagement mechanism releasably coupling the first ring gear equivalent to the output element with the first rotation element composed of the second sun gear, a fourth engagement mechanism releasably coupling the first carrier equivalent to the input element with the first rotation element composed of the second sun gear, a fifth engagement mechanism releasably fixing the first rotation element composed of the second sun gear to the transmission case, and a sixth engagement mechanism releasably fixing the second rotation element composed of the second carrier to the transmission case.
According to the configurations mentioned above, a first gear speed is established when the first engagement mechanism and the sixth engagement mechanism are engaged; a second gear speed is established when the first engagement mechanism and the fifth engagement mechanism are engaged; a third gear speed is established when the first engagement mechanism and the third engagement mechanism are engaged; a fourth gear speed is established when the first engagement mechanism and the fourth engagement mechanism are engaged.
A fifth gear speed is established when the first engagement mechanism and the second engagement mechanism are engaged; a sixth gear speed is established when the second engagement mechanism and the fourth engagement mechanism are engaged; a seventh gear speed is established when the second engagement mechanism and the third engagement mechanism are engaged; an eighth gear speed is established when the second engagement mechanism and the fifth engagement mechanism are engaged. In addition, if the first gear speed or the eighth gear speed is excluded, it is possible to establish 7 forward gear speeds.
In the conventional transmission, the engagement mechanisms to be engaged in each gear speed are two. Thereby, the dragging among the other four freed engagement mechanisms will make friction loss become greater, deteriorating the efficiency of the automatic transmission.